Their infections were not caused by any of the known human malaria species however, but by the simian malaria parasite Plasmodium knowlesi. The definitive review article on this very interesting cause of zoonotic malaria is available here. It was fascinating to learn that P. knowlesi was also initially used as malariotherapy for the treatment of neurosyphilis in the years before penicillin was discovered (P. vivax was more commonly used, and is well known for this historical purpose), but its use was discontinued when researchers found that the parasite became more pathogenic after multiple passages in humans with neurosyphilis.